1. Field of the Invention
The present invention relates to a method for producing an electric-resistance-welded steel pipe, and more particularly to a method for producing an electric-resistance-welded steel pipe using a high density energy beam.
2. Description of the Related Arts
In conventional methods, a steel strip is formed into a cylindrical shape while transferring the steel strip so that both edges of the steel strip face each other. The both edges of the steel strip are welded to produce a steel pipe. Among the conventional methods, an electric-resistance-welding method is the most efficient. In the electric-resistance-welding method, a butt joint portion is heated and fused to form a fusion zone by using induction heating of high frequency electric current or applying a high frequency electric current to the butt joint portion from a contact chip. The fusion zone is pressed by squeeze rolls to remove impurities and oxides which exist in the fusion zone, thus the welding being done. However, oxides having high melting point are not removed and remain in a weld zone by oxygen in air during the heating and the fusion of the edges of the steel strip. The oxides comprise alloying elements such as Mn, Si, and Cr. The remained oxides may result in a weld defect called the "penetrator". To suppress oxidization and to obtain a high quality welded steel pipe, gas shielding is applied. The gas shielding, however, gives not sufficient effect.
A pipe-producing method using a high density energy beam such as laser beam has been developed on the background described above. The method is a welding method to fuse the edges of a steel strip under irradiation of high density energy beam. It is known that since the method avoids exposing the fused metal to atmospheric air, weld defects such as oxide inclusion are hard to generate, and high quality welded steel pipe is obtained.
The method using high density energy beam is disclosed in JP-A-3-291176 which is published on Dec. 20, 1991 and JP-B-4-18954 which is published Mar. 30, 1992 (the terms "JP-A-" and "JP-B" referred to herein signify "unexamined Japanese patent publication" and "examined Japanese patent publication", respectively). FIG. 4 shows a pipe-producing method disclosed in JP-A-3-291176. A steel strip 1 is formed into an open pipe having edges 1a and 1b. The edges 1a and 1b are heated to a temperature of from 200 to 600.degree. C. using a heat source 2 of a high frequency induction method or a high frequency resistance method. Then a high density energy beam as the second heat source 3 is radiated to a portion 1c adjacent to squeeze rolls 4a and 4b to weld the portion. The method is a complex welding method which increases the welding speed by preheating the steel strip edges using the first heat source to make up the insufficient energy of the second heat source.
The above-mentioned method aims to produce ferritic or austenitic stainless steel pipe. But secondary operation properties such as cutting, bending, press forming and forging deteriorate owing to coarsening of crystal grains in the ferritic stainless steel. The quality of the steel pipe deteriorates owing to weld decay in the austenitic stainless steel. For the above-mentioned problems, the preheating by the high frequency heat source 2 is limited to a temperature of from 200 to 600.degree. C. If, however, a size of steel pipe or a wall thickness of steel pipe increases, the increase of the welding speed cannot be expected by the preheating of a temperature of around 600.degree. C.
FIG. 5 shows a pipe-producing method disclosed in JP-B4-18954. According to the pipe-producing method, preheating is carried out by using the induction heating coil 2 as the first heat source. Upstream of a V-converging point 1d where both edges 1a and 1b of the steel strip firstly contact each other is preheated to a degree that very little fusion occurs at the edges 1a and 1b of the steel strip. The joint face 1c of the steel strip at downstream of the V-converging point 1d is welded by radiating a high density energy beam 3 as a second heat source. In a portion between the V-converging point 1d and the fusion zone by the high density energy beam, and in a portion downstream of the fusion zone, a clamping force is applied to both edges of the steel strip to a degree that both edges simply touch each other.
In this method, the place of irradiation of high density energy beam 3 is carried out at downstream of the V-converging point 1d where both edges of the steel strip contact each other for the first time. As shown in FIG. 5, however, the joint portion may open owing to spring back at downstream of the squeeze rolls 4a and 4b to induce undercut at the weld zone, or solidification cracks may generate depending on the composition of the material. The squeeze rolls 4a, 4b and the squeeze rolls 5a, 5b clamp both edges of the steel strip to a degree that both edges are simply touch each other. Such clamp of both edges, however, has been hard to technologically attain, since there appears a gap between the squeeze rolls 4a and 4b and the squeeze rolls 5a and 5b caused by spring back.